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BackgroundThe cause of the motor neuron (MN) death that drives terminal pathology in Amyotrophic Lateral Sclerosis (ALS) remains unknown, and it is thought that the cellular environment of the MN may play a key role in MN survival. Several lines of evidence implicate vesicles in ALS, including that extracellular vesicles may carry toxic elements from astrocytes towards motor neurons, and that pathological proteins have been identified in circulating extracellular vesicles of sporadic ALS patients. Since MN degeneration at the neuromuscular junction is a feature of ALS, and muscle is a vesicle-secretory tissue, we hypothesized that muscle vesicles may be involved in ALS pathology.MethodsSporadic ALS patients were confirmed to be ALS according to El Escorial criteria, were genotyped to test for classic gene mutations associated with ALS, and physical function was assessed using the ALSFRS-R score. Muscle biopsies of either mildly affected deltoids of ALS patients (n=27) or deltoids of aged-matched healthy subjects (n=30) were used for extraction of muscle stem cells, to perform immunohistology, or for electron microscopy. Muscle stem cells were characterized by immunostaining, RTqPCR and transcriptomic analysis. Secreted muscle vesicles were characterized by proteomic analysis, Western blot, NanoSight, and electron microscopy. The effects of muscle vesicles isolated from the culture medium of ALS and healthy myotubes were tested on healthy human-derived iPSC motor neurons and on healthy human myotubes, with untreated cells used as controls.ResultsAn accumulation of multivesicular bodies was observed in muscle biopsies of sporadic ALS patients by immunostaining and electron microscopy. Study of muscle biopsies and biopsy-derived denervation-naïve differentiated muscle stem cells (myotubes) revealed a consistent disease signature in ALS myotubes, including intracellular accumulation of exosome-like vesicles and disruption of RNA-processing. Compared to vesicles from healthy control myotubes, when administered to healthy motor neurons the vesicles of ALS myotubes induced shortened, less branched neurites, cell death, and disrupted localization of RNA and RNA-processing proteins. The RNA-processing protein FUS and a majority of its binding partners were present in ALS muscle vesicles, and toxicity was dependent on the expression level of FUS in recipient cells. Toxicity to recipient motor neurons was abolished by anti-CD63 immuno-blocking of vesicle uptake.ConclusionALS muscle vesicles are shown to be toxic to motor neurons, which establishes the skeletal muscle as a potential source of vesicle-mediated toxicity in ALS.One Sentence SummaryMuscle cells of ALS patients secrete vesicles that are toxic to motor neurons

Isaxonine phosphate or Nerfactor (2-isopropylaminopyrimidine) has been implicated in several cases of hepatitis which is reversible after withdrawal of the drug. In order to understand the cause of such hepatitis, the metabolic activation of isaxonine phosphate with different liver microsomes was investigated. The major metabolites were 5-hydroxyisopropylaminopyrimidine and 2-aminopyrimidine. Covalent binding to microsomal proteins was also detected. In vitro metabolic activation required intact microsomes, NADPH and O2 as cofactors and was cytochrome P-450 dependent. A sensitive fluorimetric assay of 5-hydroxyisaxonine was developed. The metabolism of isaxonine phosphate was compared in liver microsomes from rat, rabbit, dog, monkey and man and found to be qualitatively similar. Treatment of rats with phenobarbital increased the formation of 5-hydroxyisaxonine, while treatment with 3-methylcholanthrene increased the formation of 2-aminopyrimidine but decreased that of 5-hydroxyisaxonine. Inhibition and reconstitution experiments demonstrated that 5-hydroxylation of isaxonine was catalyzed by a cytochrome P-450. Metabolic oxidation of isaxonine phosphate using 5-[3H]isaxonine phosphate led to a total loss of tritium in 5-hydroxyisaxonine and partial loss of tritium in 2-aminopyrimidine and covalent binding to proteins.